Objective lenses are well known and provide the final focusing element in an optical train of components in an optical system such as a micro-machining apparatus for using a laser beam to micro-machine a workpiece. Achromatic objectives which focus only two wavelengths at the same point are known. Apochromatic objectives which focus three wavelengths of light at the same point are also known. The lens disclosed in IBM Technical Disclosure Bulletin, Vol. 33, No. 04, September 1990, Eximer Laser Lens, pages 206-207 is apochromatic between the ultraviolet and the visible wavelengths of light, however only achieves achromatic correction for two wavelengths in the visible band.